The continuous shrinkage of microelectronic devices such as capacitors and gates over the years has led to a situation where the materials traditionally used in integrated circuit technology are approaching their performance limits. Silicon (i.e., doped polysilicon) has generally been the substrate of choice, and silicon dioxide (SiO2) has frequently been used as the dielectric material with silicon to construct microelectronic devices. However, when the SiO2 layer is thinned to 1 nm (i.e., a thickness of only 4 or 5 molecules), as is desired in the newest micro devices, the layer no longer effectively performs as an insulator due to the tunneling current running through it.
Thus, new high dielectric constant materials are needed to extend device performance. Such materials need to demonstrate high permittivity, barrier height to prevent tunneling, stability in direct contact with silicon, and good interface quality and film morphology. Furthermore, such materials must be compatible with the gate material, electrodes, semiconductor processing temperatures, and operating conditions.
High quality thin oxide films of metals, such as ZrO2, HfO2, Al2O3, and YSZ deposited on semiconductor wafers have recently gained interest for use in memories (e.g., dynamic random access memory (DRAM) devices, static random access memory (SRAM) devices, and ferroelectric memory (FERAM) devices). These materials have high dielectric constants and therefore are attractive as replacements in memories for SiO2 where very thin layers are required. These metal oxide layers are thermodynamically stable in the presence of silicon, minimizing silicon oxidation upon thermal annealing, and appear to be compatible with metal gate electrodes. Specifically, for gate dielectrics, La2O3, HfO2, and ZrO2 are also promising as they possess relatively high values for permittivity and bandgap.
This discovery has led to an effort to investigate various deposition processes to form layers, especially dielectric layers, based on metal oxides. Such deposition processes have included vapor deposition, metal thermal oxidation, and high vacuum sputtering. Vapor deposition processes, which includes chemical vapor deposition (CVD) and atomic layer deposition (ALD), are very appealing as they provide for excellent control of dielectric uniformity and thickness on a substrate. But vapor deposition processes typically involve the co-reaction of reactive metal precursor compounds with an oxygen source such as oxygen or water, either of which can cause formation of an undesirable SiO2 interfacial layer. Thus, an effort is underway to develop water- and oxygen-free vapor deposition processes.
Ritala et al., “Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources,” SCIENCE, 288:319–321 (2000) describe a chemical approach to ALD of thin oxide films. In this approach, a metal alkoxide, serving as both a metal source and an oxygen source, reacts with another metal compound such as a metal chloride or metal alkyl to deposit a metal oxide on silicon without creating an interfacial silicon oxide layer. However, undesirable chlorine residues can also be formed. Furthermore, zirconium and hafnium alkyls are generally unstable and not commercially available. They would also likely leave carbon in the resultant films.
Despite these continual improvements in semiconductor dielectric layers, there remains a need for a vapor deposition process utilizing sufficiently volatile metal precursor compounds that can form a thin, high quality oxide layer, particularly on a semiconductor substrate using a vapor deposition process.